Any rite of passage that involves jabbing needles into small children is bound to worry more than a few parents. But that doesn't begin to explain why so many moms and dads are convinced  despite mounting scientific evidence to the contrary  that the triple vaccine against measles, mumps and rubella (MMR) causes autism in some youngsters. The latest study exonerating the MMR vaccine comes from Denmark, where investigators looked at the health records of every child born from 1991 through '98, more than 537,000 children. No matter how researchers analyzed the data, there was no difference in the autism rates of children who received the MMR vaccine and those who did not.

The Danish findings, which were published in the New England Journal of Medicine last week, are persuasive for several reasons. Denmark's socialized medical system has generated one of the most complete health records of any country. So the investigators were able to document accurately both sides of the equation: those who were (or were not) vaccinated and those who developed autism. Even when other factors, such as age at vaccination, were taken into account, there was no difference in autism rates between vaccinated and unvaccinated children. There was no clustering of autism diagnoses in the weeks and months after vaccination. There was no difference in the number of diagnoses of other developmental disorders related to autism in the vaccinated and unvaccinated groups.

Other epidemiological studies over the past four years have come to similar conclusions, but none has been so large and so complete as the Danish study. Indeed, the accumulated evidence is strong enough to convince even onetime proponents of the MMR-autism link, like Dr. Jeff Bradstreet, director of the International Child Development Resource Center in Palm Bay, Fla. "MMR does not appear to cause autism," Bradstreet concedes. "If it did, it would be a godsend because we could change the vaccine and that would be it." Still, he suspects that the MMR vaccine might worsen a pre-existing autistic condition.

The evidence for even that tenuous link is hotly debated. "If MMR made autism worse, then we would expect to see different rates [between vaccinated and unvaccinated children] in cases of both autism and related disorders," says Dr. Kreesten Madsen, the epidemiologist who led the Danish study. But that difference did not show up.

More and more, it seems as though the focus on the MMR vaccine has been a colossal distraction in autism research  and in parental concern. Just as a few eyewitness reports made in good faith led police to focus on a white van in the search for the Beltway snipers and overlook the blue Caprice, the controversy over MMR may have prompted parents of autistic children to focus too intently on vaccination. The latest research suggests that the disorder begins in the womb  long before any vaccines are given. There is also intriguing evidence of abnormalities in the immune system. But there is no evidence that the MMR vaccine causes autism.